Skid control system

ABSTRACT

A braking system including a skid control system, in which the brakes are actuated from a source of hydraulic power with the magnitude of pressure to the brakes being varied generally as a function of the input force from the vehicle operator and; during controlled operation, as a function of the magnitude of an error signal indicative of an incipient skid or excessive slip condition. The pressure is varied by a valve system including a pilot valve receiving hydraulic pressure from the source of hydraulic power which is actuatable by the vehicle operator and by an electromotive means responsive to the error signal to provide hydraulic pressure variations over a first predetermined range of pressures. The valve system further includes a main valve connected to the actuators for the brakes which is responsive to the first range of pressures from the pilot valve so as to provide hydraulic pressure variations over a greater range of pressures for operating the brakes.

United States Patent Riordan 1 Aug. 29, 1972 I54! SKID CONTROL SYSTEMPrimary ExaminerMilton Buchler [72] Inventor: Hugh E. Riordan, AnnArbor, Mich. Ass'stam Exam'Mrs.tephen K AttorneyHarness, Dickey andPierce [73] Assignee: Kelsey-Hayes Company, Romulus,

Mlch- 57 ABSTRACT Filed! g- 3, 1970 A braking system including a skidcontrol system, in [21] APPL N03 60,273 which the brakes are actuatedfrom a source of hydraulic power with the magnitude of pressure to thebrakes being varied generally as a function of the [52] US. Cl. ..303/2lF, 303/10, 303/21 AF, input force f om h hi le operator and; during con-303/63 trolled operation, as a function of the magnitude of an [51][111. CI. ..B60l 8/08, B601; 8/10 error Signal indicative of anincipient Skid or excessive Fleld of Search l 52, condition The pressureis varied a valve system .I 9i l 0, 13, 84 including a pilot valvereceiving hydraulic pressure from the source of hydraulic power which isactuata- [56] References C'ted ble by the vehicle operator and by anelectromotive UNITED STATES PATENTS means responsive to the error signalto provide hydraulic pressure vanatlons over a first predeter- Stelzer Fmined range of pressures The valve ystem further in- 3,401,982 1968 l etF cludes a main valve connected to the actuators for the Helmler Fbrakes is responsive to the first range of pres- 3,532,394 10/1970Goddard ..303/21 F sures f the pilot valve so as to provide hydraulic3,539,227 11/1970 Drutchas et "303/ 21 F pressure variations over agreater range of pressures 3,556,608 1/ 1971 MacDuff et a1. ..303/21 F fOperating the brakes 27 Claims, 11 Drawing Figures iii/ PATENTEDwszsI972 SHEET 1 OF 6 02% l l i/im) Fear INVENTOR HUGH R. RIORDAN ATTORNEYPAIENTEDAUGZQ I912 33587. 504

SHEET 3 BF 6' INVENTOR E m R. mm

ATTORNEY SKID CONTROL SYSTEM SUMMARY BACKGROUND OF THE INVENTION Thepresent invention relates to brake control systems and more particularlyto a system in which the brakes are controlled via hydraulicamplification in which the output brake pressure is generallyproportional to the input force from the vehicle operator and in whichthe brake pressure can be continuously modulated and controlled for skidcontrol.

When the wheels of a vehicle are in an incipient skid condition, thebraking torque or pressure is excessive and must be reduced in order toavoid a locked wheel condition. Generally in some skid control systems,the brake pressure is alternately relieved and re-applied to avoid alocked wheel condition. In these systems, however, while a locked wheelcondition may be avoided resulting in good braking performance, stillthe stopping distance may not be optimized because of the extensiveswings or variations in wheel speed during the control cycles, i.e. fromnear locked wheel to synchronous speed. In the present invention thebrake pressure is controlled such as to reduce the magnitude of theexcursions of the wheel speed during skid control and to maintain theexcursions of the wheel speed small and oscillating generally around apreselected magnitude of desired wheel speed during a brake stop; thisspeed generally, continuously decreases in magnitude during the brakestop. Therefore it is an object of the present invention to provide anew and improved skid control system in which the wheel deviationsduring skid control are controlled and are maintained small in magnitudeduring braking. The present system utilizes a hydraulic amplifyingdevice for the main brake controller and this controller, for normaloperation, is actuated by the vehicle operator and provides an outputbrake pressure generally proportional to the force input by the vehicleoperator; at the same time the system can be skid controlled to vary theoutput pres sure substantially independently of the input force. Thesystem leads itself to a continuously modulated skid control in whichthe response is generally proportional to the magnitude of an errorsignal It is another general object of the present invention to providea new and improved brake control system and further object to providesuch a system which can readily be adapted for use with a skid controlsystem.

Other objects, features, and advantages of the present invention willbecome apparent from the subsequent description and the appended claims,taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a curve showing wheel and vehicle speed versus time forexemplifying features of the system including the present invention;

FIG. 2 is a general schematic of a vehicle with a skid control systemincluding features of the present invention;

FIGS. 3 A and B taken together depict block diagram for an electroniccontrol circuit for controlling a hydraulically actuated brake systemfor the system of FIG. 2;

FIGS. 3 C and 3 D are modified forms of the control circuit of FIGS. 3 Aand 3 B;

FIG. 4 is a block diagram of the hydraulic system for the system of FIG.2 and depicting the hydraulic portion of the control system of thepresent invention;

FIG. 4 A is a modified form of fluid separator for the system of FIG. 4;

FIG. 5 is a side elevational view, with some parts shown in section, ofthe control valve of the system of FIG. 3;

FIG. 6 is a fragmentary view of a modified form of the control valve ofFIG. 5; and

FIG. 7 is a fragmentary view of another modified form of the controlvalve of FIG. 5.

Looking now to FIG. 1, a curve 10 shows the relationship of wheel speedw versus time t and a curve 12 shows the actual vehicle speed Vv versustime during a brake stop. Since the actual speed cannot be convenientlymeasured directly the vehicle speed is simulated (by means to bedescribed) and the simulated speed and measured wheel speed are utilizedto determine slip. The simulated vehicle speed Va is indicated by thecurve 14. In considering the brake system of the present invention,first its operation as a skid control system will be considered.

SKID CONTROL SYSTEM The skid control system utilized in conjunction withthe present invention has substantially two modes of operation referredto as modes A and B. The normal braking mode just prior to braking andduring the initial stages of run down of the wheels during initialbraking is shown as mode B. Upon the occurrence of an incipient skidcondition a skid control signal occurs placing the system in mode A. Theoperation of modes A and B will be described generally and this willthen be followed with a more detailed description.

In the system shown, there is continuously generated a wheel speedsignal w and a wheel acceleration or deceleration (depending on sign)signal w. This acceleration signal w is continuously compared with aselected reference signal which is different for modes A and B. In anidealized brake system, braking would be maximized by maintaining thewheel at a speed providing a selected amount of slip relative to vehiclevelocity. To accomplish this it is desirable to minimize the variationin wheel velocity from that velocity providing the desired amount ofslip. Towards this objective then it is desirable to control wheeldeceleration prior to the occurrence of an incipient skid condition soas to prevent excessive wheel decelerations which may result in thewheel running down too far from synchronous speed and too close to alocked wheel condition before controlled correction can be attained. Forexample in panic brake stops it is possible for the wheel to decelerateat a rate as high as 15 20 gs on low mu surfaces and substantially lesson high mu surfaces. An excessive deceleration of the wheel will tend toprovide excessive wheel slip before controlled correction can occur.This, of course, prevents braking from being optimized. By limiting thedeceleration rate at initial braking, the wheel velocity will be heldcloser to the desired wheel velocity at braking to provide the desiredslip and hence less correction will be required. Thus in the presentinvention for an automotive application it was found desirable to limitwheel deceleration -w at initial braking at around a 3 5 g decelerationrate. Thus in mode B the system is preset to control (relieve) brakepressure such that wheel deceleration should not exceed 5 gs.

The system is selected to sense slip, i.e. vehicle velocity minus wheelvelocity, and to determine when this attains a selected magnitude whichis indicative of an incipient skid condition. When this occurs a skidcontrol signal x is generated indicating that additional brake controlis required and the system is switched to mode A. While mode Bcontrolled the initial wheel deceleration to prevent excessivedeceleration and hence excessive run down, the mode A phase is designedto control spin-up rate.

In mode A a reference signal indicative of a lg acceleration is used andis compared to the wheel acceleration (or deceleration) signal w.Initially in mode A the wheel is still decelerating and the measuredrate is compared to the positive acceleration reference to provide a neterror signal which will require a substantial decrease in brakepressure. Since the wheel deceleration has already been controlled inmode B, the amount of correction required in mode A need not be soextensive resulting in a controlled rapid decrease in magnitude of wheeldeceleration. When the wheel begins to spin-up it is desirable to holdthe spin-up rate to the relatively low magnitude of around lg. Againwhere wheel spin-up is uncontrolled the wheel can under high muconditions rapidly approach and reach synchronous speed and be at thatspeed for an excessive period of time before the re-application ofpressure can be effective; this would preclude stopping distances frombeing optimized. Thus in the system of the present invention the spin-uprate is held to around lg with the brake pressure modulated, i.e.increased to decreased, to maintain that acceleration rate in accordancewith the magnitude of an error signal which is based upon the differencebetween the magnitude of acceleration and the selected lg rate. Near theend of the cycle as wheel speed approaches synchronous, the accelerationtends to fall off; however, at this point and also to minimize runningat synchronous the mode A phase of operation is clamped such that brakepressure cannot be decreased but only increased so that wheelacceleration is at lg or less. The system of the present invention willat a preselected low magnitude of acceleration (less than lg) switchback to original mode B; at this low magnitude of acceleration wheelspeed is near synchronous. In mode B the 5g reference is again appliedand with the wheel at some low magnitude of acceleration, the resultwill be a signal calling for an increase of brake pressure to deceleratethe wheel towards the 5g level. The cycle repeats itself as long as theskid condition continues.

Looking now to FIG. 2 a vehicle is shown to have a pair of front wheels22 and a pair of rear wheels 24. A hydraulic brake system 26 has a fluidactuator with a first fluid line 28 connected to front brake cylinders30 and with a second fluid line 32 connected to rear brake cylinders 34.The fluid actuator 25 is actuated by the vehicle operator via a footpedal and linkage assembly 36. The fluid actuator 25 can also beactuated to control the brake pressure to the fronts and rears inresponse to signals from a skid control electronic module 38 viaconductors 40 and 42. The module 38 receives wheel speed signals fromfront wheel sensors 44 via conductors 46 and from rear wheel sensors 48via conductors 50.

Looking now to FIGS. 3A and 3B, the module 38 is shown and receives thesignal from the rear wheel sensors 48 via conductors 50. The signals arefed to a tachometer circuit 53 which integrates the signals to provide ad-c output signal w the magnitude of which is indicative of themagnitude of the combined rear wheel speeds.

As previously noted the mode of operation, i.e. whether mode A or mode3B, is significant to the operation of system and this is detected bymode circuit 52 which receives information from vehicle ramp circuit 54and a slip threshold circuit 56. The wheel velocity signal w istransmitted to the slip threshold circuit 56 via conductor 58. Thecircuit 56 utilizes the magnitude of wheel velocity signal w to generatean adjusted threshold wheel velocity signal wa which appears atconductor 60. The threshold signal wa is compared to the simulatedvehicle velocity signal Va at conductor 62 via a comparator circuit 64in the mode circuit 52 and when Va exceeds wa an output slip signal xwill be generated by circuit 64.

The simulated vehicle velocity signal Va is derived from the wheelvelocity signal w which is transmitted to the ramp circuit 54 viaconductor 63.

VEHICLE RAMP CIRCUIT 54 Circuit 54 can simply comprise a capacitor whichnormally is charged through a diode to a potential generally equal tothat of wheel speed signal w and hence will normally provide anindication'of wheel speed. When no braking occurs this potential is alsoindicative of actual vehicle velocity Vv. The circuit 54 is providedwith a discharge circuit having a time constant indicative of a selectedvehicle deceleration rate. When the brakes are applied and the wheelspeed signal w drops below the magnitude of the charge on the capacitor,the capacitor will start to discharge at a selected rate and provide anoutput signal Va which will have a magnitude approximating vehiclevelocity. The ramp 54 will be reset each time the wheels spin-up toprovide a signal w to have a magnitude greater than the charge in thecapacitor or greater than simulated signal Va. To assure resetting thedischarge rate of the capacitor is selected to simulate a decelerationrate greater than the maximum attainable; thus simulated vehiclevelocity Va will always be less than actual vehicle velocity Vv.

During braking, there is a certain amount of slip and in normal brakingthe slip is not excessive; it is excessive when an incipient skidcondition occurs which should be corrected be relieving brake pressurein order to avoid a locked wheel condition. This factor is determinedand is used in the present system in a manner to be seen.

SLIP THRESHOLD CIRCUIT 56,

The wheel velocity w at conductor 58 is multiplied by a factor a atvoltage divider circuit 66 to provide a signal aw which is a fixedpercent of w and which is connected to addition circuit 68 via conductor69. To the signal aw a fixed amount of slip w is added to additioncircuit 68 via conductor 71. This combined signal aw Aw) is added viaconductor 73 to wheel velocity signal w at addition circuit 70. Notethat the slip threshold signal wa then equals wheel speed w plus thefixed slip Aw plus a variable slip quantity aw. This total wa iscompared to the simulated vehicle velocity Va and when Va exceeds wa itwill mean that Va exceeds wheel speed w by the sum of (aw Aw) and thatsum is selected as indicative of excessive slip which occurs in anincipient skid condition. The sum of (aw Aw) will vary with wheel speedsuch that the slip signal x will be generated at higher wheel speeds wfor higher vehicle velocities Va and at lower wheel speeds w for lowervehicle velocities Va; this also assures that at low vehicle speeds(when w max and aw will be small) that the fixed slip signal Aw will beof sufficient magnitude to provide the desired slip threshold wa. Theslip signal x is a logical 1 when circuit 64 is on (i.e. excessive slipdetected) and a logical 0 when circuit 64 is off (i.e. slip notexcessive).

As previously noted the slip signal x isused to control the condition ofthe mode circuit 52 and when signal x occurs the mode circuit 52 will beswitched from B (initial braking) to mode A (incipient skid).

MODE CIRCUIT 52 As previously discussed, the brake control system hastwo modes, mode A and mode B and in each mode a different accelerationor deceleration reference; at the same time the brake pressure ismodulated in accordance with the magnitude of an error signal based uponthe magnitude of the difference between wheel deceleration and theacceleration or deceleration references.

The mode circuit 52 includes a final flip-flop circuit 76 which providesan output signal Y at output conductor 78 which is a logical l in mode Aand a logical 0 when in mode B. The flip-flop 76 has an on input 80 andan off input 82 and the on input being an overriding input which willcontrol in the event signals simultaneously occur at inputs 80 and 82.The overriding input 80 receives its signal Z from the output 84 of anintermediate flip-flop circuit 86. the signal Z is a logical l whenflip-flop 86 is in mode A and 0 when in mode B. The use of a pair offlip-flops 76 and 86, as will be understood from subsequent description,is to permit the control circuit to be placed in a modified form of themode A operation; this modified form is designated mode A and will belater described.

The intermediate flip-flop 86 has an overriding on input 88 and an offinput 91. While the intermediate flip-flop 86 is used to control finalflip-flop 76 (via signal Z), its primary purpose is to detect and signalwhen mode A is to occur.

The input 88 is actuated via a signal U from the output of an Or circuit90 which has two inputs one of which is connected to the comparatorcircuit 64 to receive the slip signal x. The flip-flop 86 will normallybe off or in the B mode and will be switched on or to the A mode whenslip is excessive, i.e. Va exceeds wa, and signal x is a logical l. Theflip-flop 76 will also normally be of or in the B mode and will beswitched on" or to the A mode in response to signal Z being a logical 1In accordance with the prior discussion, the system will be in the Amode (or A mode) to control spin-up and will remain on that mode untilthe wheel acceleration w reaches a selected minimum value indicative ofa condition at which the wheels are approaching synchronous. Thus thewheel speed signal w at conductor 58 is connected to the input of adifferentiating circuit 92, which will provide an output signal w whichis an indication of the acceleration of the wheel. This signal w istransmitted to a comparator circuit 94 via conductor 96 and there iscompared to a selected threshold W which is representative of theminimum acceleration referred to. When w decreases below w whencomparator circuit 94 will be actuated to generate the turn off signalR, which is transmitted to the off input 82 of flip-flop 76. Assuming nosignal Z at on input 80, then signal R will switch flip-flop 82 to itsoff condition placing it back into the B mode.

Some mention should be made of the mode A, and the curves of FIG. 1should be referred to. With the system in mode A the brake pressure ismodulated to maintain wheel acceleration at around lg. When, however,the simulated vehicle speed Va ramp curve 14 intersects the wheel speedw, curve 10 the slip signal x will no longer be generated since it willhave been terminated slightly earlier when speed w and slip thresholdexceeded the vehicle speed Va. In this region it is desirable to holdthe wheel acceleration to lg or less but not permit acceleration toexceed lg. In doing this the wheels are controlled so that the time atwhich they are near synchronous speed is minimized. This is done bypermitting error signals to be generated which will provide only holdingof brake pressure or increases in brake pressure. It is significant togood brake control that the point of intersection be locatedaccurately.. This can be done by differentiating the simulated vehiclespeed signal Va. From FIG. 1 it can be seen that the curve 14 decreasesin magnitude at a relatively uniform slow, rate until it intersects thewheel speed curve 10 and at that point curve 14 and curve 10 are thesame and hence curve 14 now increases with time. A differentiatorcircuit 97 receives the vehicle velocity signal Va via conductor 98connected to ramp section 54 via conductor 62. Circuit 97 will providean output Va only in response to derivatives of decreases in vehiclevelocity Va. The derivative signal Va is fed to a comparator 100 viaconductor 102 and compared to a threshold Va and when Va exceeds (isless negative than) Va then an off signal Q will be transmitted to offinput of flip-flop 86 switching flip-flop 86 off. In this conditionflip-flop 86 will be in its condition for the B mode and flip-flop 76will be in its condition for the A mode and in that situation the mode Aexists and will be utilized in a manner to be described.

Looking to the mode circuit 52, two output conduc tors, 78 fromflip-flop 76 and 104 from flip-flop 86 via conductor 84, exist. Thus,for mode A the Y and Z signals will appear at conductors 78 and 104,respectively; for mode B neither the Y or Z signal will appear; and formode A the Y signal will appear at conductor 78 while the Z signal willnot appear at conductor 104. The above logic signals from the modecircuit 52 are used to control the acceleration and decelerationreferences used in the generation of the error signal and hence thesesignals are connected to the error signal generator circuit 106.

ERROR SIGNAL GENERATOR 106 The output signal Y is transmitted to theinput of reference signal generator 108 which has an output conductor110. The reference generator 108 will provide an acceleration referenceoutput signal M having a magnitude selected to be representative of a lgacceleration rate in response to the signal Y appearing at conductor 78.The generator will provide a deceleration reference, output signal Nhaving a magnitude selected to be representative of a 5g decelerationrate in response to the signal Y not appearing, i.e. a logical 0, atconductor 78. An error signal A w is generated by subtracting the wheeldeceleration (or acceleration) w from the reference signal M or N. Thusa subtracting circuit 112 receives the reference signal (M or N) fromreference generator 108 via conductor 110 and the wheel deceleration(acceleration) signal w from differentiator circuit 92 via conductor 114and provides the error signal Aw at its output conductor 116. As will beseen a negative error signal (Aw) will result in an increase in brakepressure and a positive error signal (+Aw) will result in a decrease inbrake pressure.

The error signal Aw is transmitted to mode A selector circuit 120 viaconductor 116. Selector 120 is normally in a condition to transmit theerror signal Aw directly to output conductor 122 via conductor 124.However, in mode A, selector 120 is actuated to switch the output signalAw to output conductor 122 via diode D1. Diode D1 will permit onlynegative magnitudes of Aw to be transmitted and hence, in accordancewith the prior description, mode A will control such that only increasesin brake pressure can occur. The mode A selector is actuated by a signalL at conductor 126 from OR circuit 128. OR circuit 128 has one inputconnected to conductor 104 (from flip-flop 86) via invertor circuit 130.In either the A or B modes OR circuit 128 will be actuated to generatethe L signal and to hold A mode selector in its normal condition, i.e.circuit of conductor 124 actuated; in the A mode the OR circuit 128 willnot be actuated and L signal will not appear, i.e. at logic and circuit120 will be switched to its A mode, i.e. circuit of diode D1 actuated.The output error signal Aw at conductor 122 will be transmitted to theproportional controller circuit 132 which provides the final outputsignal to control the brake controller or actuator 25.

Note that in mode B the vehicle wheels will be decelerating and thiswheel deceleration will be compared with the 5g deceleration referenceto provide the error signal Aw. Before the wheel begins to accelerate,the mode circuit 106 will be switched to the A mode which has a +lgreference; initially in the A mode, however, the wheels will still bedecelerating. This will result in a larger Aw indicating thatsubstantial relief in brake pressure is required; in other words, thedifference in sign between wheel deceleration and acceleration referenceis taken into account and will result in the large Aw. As the wheelbeings spin-up then Aw can decrease and may even change sign if the +lgacceleration is exceeded. To complete the cycle, the mode A operationpermits only holding of or increases of brake pressure to hold spin-upto lg or less after the slip signal x is gone; and finally when theacceleration of the wheels drops below a selected minimum (w') then themode circuit is again switched to its mode B operation.

PROPORTIONAL CONTROLLER CIRCUIT 132 This circuit 132 has an integratingcircuit 134 and a proportional circuit 136. The error signal Aw is actedupon by both of these circuits to provide the final error signal dw. Theintegrating circuit 134 provides a signal Swdl which represents themagnitude of correction over a selected time period required to bringthe brake pressure to provide the desired (reference) acceleration ordeceleration and when the wheel deceleration (ac-- celeration) is at thedesired magnitude the integral signal Swd b will represent the changerequired from the brake pressure established by the vehicle operator tomaintain the reference acceleration or deceleration. UntiL'however, thisdesired magnitude is obtained the integral signal Swdiv will besupplemented by a proportional signal pw. This provides an instantaneousindication of the magnitude of the correction still required to attainthe desired wheel deceleration (acceleration). When this acceleration(deceleration) has been attained then the proportional signal pw will bezero and at this time the final output signal dw will equal the integralsignal Swdt.

By varying the ratio of proportional signal pm? to integral signal Swatlead time can be built in to the system to accommodate the lag ofvarious electrical and mechanical com-- ponents of the system.

Looking now to integrator circuit 134, error signal Aw is multiplied bya selected constant Kl at multiplier 138 and the product is transmittedto add circuit 140 via conductor 142. The output from add circuit 140 isintegrated by integrator 144 (via conductor 146) to provide integratoroutput summing circuit 154 where the two signals are added to providethe final output signal dw. The integrated signal Swdt and proportionalsignal pw will vary in magnitude relative to each other and under someconditions can be equal; the relative proportions of each can be variedto accommodate different requirements for different vehicles.

For a given deviation between desired and actual wheel acceleration ordeceleration it may be that there still exists a large wheel slip; inthis condition a more rapid correction would be desired. This situationis detected by the large slip detector circuit 156.

LARGE SLIP DETECTOR CIRCUIT 156 This circuit operates in conjunctionwith the slip circuit 56 and the two should be considered together. Inslip circuit 56, the output from summing circuit 68 is transmitted viaconductor 158 to multiplier 160 where the slip signal is multiplied by afactor B (which is greater than unity). The multiplied slip signal istransmitted via conductor 162 to a summing circuit 164 and is addedthere to the wheel speed signal w. This slip signal LS is transmittedvia conductor 166 to large slip detector 156. Detector 156 has asubtraction circuit 168 which receives signal LS via conductor 166 andsubtracts it from the simulated vehicle ramp Va which it receives viaconductor 170. The output DLS is transmitted to the proportionalcontroller circuit 132 via diode D2 and conductor 172. The signal DLS ismultiplied by a factor KIS by multiplier 174 in integrating circuit 134and the multiplied output is added to the output of error signalmultiplier 138 by summing circuit 140 such that the output signal Swatwill reflect, when the large slip condition exists, the effect of largeslip signal DLS.

At the same time the large slip signal DLS is multiplied by proportionalmultiplier 176 by a factor KPS and the multiplied output is added to theoutput error signal multiplier 150 and to the output signal Swdt bysumming circuit 154 such that the final output signal dw will reflectthe effect of the large slip signal DLS.

At lower vehicle and wheel velocities, the wheel may be approachinglocked wheel and yet the wheel slip may not be sufficient to provide theslip signal x. To optimize stopping it would still be desirable toprevent lock up. This is accomplished by the anti-lock circuit 178.

ANTI-LOCK CIRCUIT 178 Since this circuit is utilized generally for lowervehicle and wheel speeds a high gain tachometer circuit 180, whichreceives wheel speed information from sensors 50 via line 184, is usedto provide a wheel speed signal wl at its output line 182. The signal W1is fed to a comparator circuit 184 where it is compared to a thresholdwl'; when w! exceeds wl an output signal La is generated and transmittedto an AND circuit 186 via line 188. The AND circuit, however, to providean antilock signal must also receive an input from a time out circuit190 which begins to time out and provide a decaying signal H in responseto the signal L at its input 192. The signal H is fed to a comparatorcircuit 194 and compared to a small threshold signal H and as long as His greater than H, comparator 194 will provide an input to AND circuit186. The time out provision is provided to prevent the occurrence of ananti-lock signal for an indefinite period. The AND circuit 186 willprovide anti-lock signal I at its output conductor 196 to OR circuit inmode circuit 52. This will result in signal U being generated by ORcircuit 90 causing flip-flop 86 and flip-flop 76 being placed in the Amode.

A modified form of the invention is shown in FIG. 3 C; it may bedesirable in some systems that, in the event the wheels are tending tolock up, the error signal Aw to the proportional controller circuit 132be increased in order that brake pressure can be quickly relieved to anever greater extent to prevent lock up. Thus in FIG. 3 C the anti-lockcircuit 178 has a second output conductor 196' connected to conductor196 and to the proportional controller circuit 132. Now the signal Ifrom AND circuit 186 is transmitted to integrating circuit 134' andmultiplied by a constant KIL at multiplier 197 with the resultant outputadded to the outputs from multipliers 138 and 174 at addition circuit Atthe same time the signal I is transmitted to proportional circuit 136'and multiplied by a constant KPL at multiplier 199 with the resultantoutput added to the other inputs to addition circuit 154. The signal Iwill have a fixed amplitude (when at logical l) and hence will add tothe integrating circuit 134 and proportional circuit 136 a fixed amountwhich is selected to initially provide a large addition to the amplitudeof the final error signal dw. The result will be a large decrease inbrake pressure to prevent wheel lock up.

FINAL OUTPUT CIRCUIT 200 The final output signal dw from proportionalcontroller circuit 132 is transmitted to a final output circuit 200 vialine 202. Signal dw is a varying voltage which is converted to avariable current via a voltage controlled current source 204. Theresultant variable current signal G is transmitted to the brakecontroller 25 via conductor 40. Note that while the controller 25 is aproportional controller other types of brake control apparatus could beutilized for providing control based upon the final output signal dw.

FRONT WHEEL CONTROL CIRCUIT 210 With the rear wheels 24 being controlledin the manner described the front wheels 22 can be controlled utilizinga substantially simplified circuit 210.

In circuit 210 the front wheel sensors 44 transmit front wheel speedsignal to a tachometer circuit 212 via conductor 46. The tachometercircuit 212 provides a d- 0 output signal wf, having an amplitude whichis indicative of the front wheel speed. The signal wf is added to afixed speed offset signal wf via summing circuit 214 to provide athreshold signal wfa. The offset signal wf' is selected as indicative ofthe difference between vehicle and front wheel speed at which excessivefront wheel slip has occurred. Thus the threshold signal wfa issubtracted from a simulated vehicle speed Vf by a difference circuit216. The vehicle speed signal Vf is derived from the simulated vehicleramp signal Va generated by the rear wheel circuit 54. The signal Vatransmitted to front wheel control circuit 210 via conductor 218 ismodified by factor E to provide the signal Vf. The front wheels arecontrolled utilizing slip only and hence the error signal for the frontwheels will be the slip signal Sf front difference circuit 216 and thefront wheels will be controlled in accordance with the magnitude of theslip signal Sf. This is done by transmitting the slip signal Sf to aproportional controlled circuit 218 via conductor 220. Circuit 218 issimilar in function to the rear wheel circuit 132 and it will provide afinal control signal Sfa which will be a combination of a signal whichis an integration of Sf, one which is proportional to Sf and one whichis a derivative of Sf. Again this is to provide memory for the totalcorrection required and also to provide the necessary lead. Thederivative component will be generally small in comparison to the othertwo components which under some conditions could be equal to each other.

The final signal Sfa is transmitted to a voltage controlled currentsource 222 via conductor 224 and there a signal Gf is generated which isa current proportional to the error signal Sfa. Again this signal istransmitted to brake controller 25 via conductor 42 to provide controlfor the front brakes.

MODIFIED FRONT WHEEL CONTROL CIRCUIT In some applications it may bedesirable to provide a different means for obtaining lead than thatshown in FIG. 3 B. FIG. 3 D shows a modified system and includes adifferent proportional control circuit 217. Cir-- cuit 217 includes afirst stage 219 which receives the slip signal Sf and provides onecomponent (SGIdt) which is the integral of the signal Sf and a secondcomponent (GP) which is proportional to the signal Sf. The sum of thesecomponents is transmitted to addition circuit 223. Note that'theproportional component (GP) provides a lead function in accordance witha fixed ratio of signal Sf. It is desirable to have a lead varying withthe rate of change of wheel speed. Thus, in FIG. 3 D the front wheelspeed signal Wf is transmitted via line 215 to a second stage 221 inproportional control circuit 217. The stage 221 differentiates Gdd/dtthe wheel speed signal Wf to provide a derivative component which willvary in magnitude with the rate of change of Wf. The derivativecomponent is added to the output from the first stage 219 at additioncircuit 223 to provide the front wheel error signal Gf. The sign of thesecond stage 221 is negative; thus when the derivative of wheel speed Wfis negative (indicative of deceleration) the output from second stage221 will be inverted and will be positive. The result is that for wheeldeceleration the error signal Gf will be increased in magnitude and foracceleration it will be decreased in magnitude. In general the outputfrom the second stage 221 will be small compared to that from the firststage 219. Also the integrated portion (SGIdt) and proportional portion(GP) will vary relative to each other and under some conditions can beequal; however, the relative proportions of each can be varied toaccommodate different requirements for different vehicles.

HYDRAULIC SYSTEM Looking now to FIG. 4, generally a block diagram of ahydraulic system capable of utilizing the control signals G and Gf forcontrolling fluid pressure to the front and rear brakes is shown.

The brake controller 25 depicted generally diagrammatically is shown tobe actuated via a rod 250 which is in turn actuated by the vehicleoperator through a conventional foot pedal. The brake controller 25 hasa control valve 252r for controlling fluid pressure to the rear brakesand 252f for the front brakes. The systems for the front and rear brake,are similar and hence only the system for the rear brakes will bedescribed.

Generally actuation of the control valve 252r via the vehicle operatorwill provide actuation of the rear brakes; however, the control valve252r will be responsive to the signal G from the control module 38. Thecontrol valve 252r controls the magnitude of fluid pressure from asource in one fluid circuit which in turn acts upon the brake fluid inseparate fluid circuit to control braking.

Thus in FIG. 4 a fluid pump 254 provides fluid flow from a source 256 toa flow divider 258 via a fluid line 260. The flow divider 258 has a rearbrake supply line 260r and a front line 260f for applying the rear andfront brake control circuits, respectively. The flow divider 258provides fluid flow at a substantially constant rate to each line 260rand 260f while permitting unequal pressure to each in accordance withthe requirements demanded. The output line 260r is connected to theinlet of the control valve 252r. The outlet of valve 252r is connectedto tank 256 via a return line 258r. The control valve 252r, in a mannerto be seen, provides a restriction to the flow to tank line 258r andthereby controls the output pressure at output line 26lr. Output line261r has a holdoff valve 262r in series therewith, with valve 262rproviding a hold-off up to a minimum pressure in order to preventunwanted continual actuation of the rear brakes at the normal, quiescentoperating pressure at line 26lr. Upon actuation of brake controller 25by the vehicle operator, the control valve 252r will be actuated torestrict flow to return line 258r resulting in an increase in pressureat output line 261r; this in turn will unseat hold-off valve 262r (whichhas one side connected to line 261r and the opposite side connected totank) permitting line 264r to transmit the pressure at line 26lr.

The line 264r is connected to a fluid separator 266r which has one sideconnected to the fluid system of pump 254 and its other side connectedto the fluid system for the brakes. In general the fluid separator has achamber divided by a diaphragm 268r with one side 270r connected tofluid line 264r for actuation by fluid from pump 25.6 and with the otherside 272r connected to the rear brake line 274r and hence to the fluidsystem for the rear brakes. Thus, the pressure to rear brakes via line274r will be controlled by the pressure applied to chamber portion 270rvia control valve 252r.

The hydraulic system has a push through capability in the event offailure of the system of pump 254. Thus a dual master cylinder 276 hasoutlets 278f and 278r to the front and rear brakes, respectively andreturns 280f and 280r to the front and rear brake reservoirs,respectively. The brake controller 25 is designed such that upon failureof the fluid system of pump 254 the rod 250 can be pushed through toactuate master cylinder 276 to provide braking in a conventional manner.A control valve 282r normally blocks output line 278r from mastercylinder 276 while permitting communication between chamber portion 272rof fluid separator 266r and brake line 274r. However, upon loss of fluidpressure from pump 254 and the actuation of master cylinder 276 thevalve 282r will be switched to block chamber portion 272r The reservoirline 280r is connected to chamber portion 272r via a check valve 284r inthe fluid separator 266r.

BRAKE CONTROLLER 25 Details of the brake controller 25 are shown in FIG.5. Thus the rod assembly 250 is connected to a flanged input cylinderassembly 290 which can move independently of and is spring coupled via alightly biased spring 292 to a cup-shaped member 294 which has an outerflange connected to a generally flat actuating plate 296. A heavy spring298 is connected between the cylinder assembly 290 and a push throughrod 300 connected to the master cylinder assembly 276. The push throughrod 300 is normally held from motion and hence the bias of the heavyspring 298 is selected to provide pedal feel to the vehicle operator.

The actuator 25 includes the control valves 252r and 252f which areidentical and hence only 252r and its associated fluid circuitry will bedescribed. The line 260r from flow divider 258 is connected to fluidpassage 261r to one side of hold-off valve 262r. Valve 262r has aconical valve member 300r held against a cooperating valve seat viaspring 302r to normally hold passage 261r closed from communication with264r until some low magnitude of pressure is generated by action of thevehicle operator. In one example valve 262r was designed to stay closeduntil a pressure of 50 psi was reached, thus preventing unwantedactuation of the vehicle brakes. The opposite side of valve member 300ris connected to tank via passage 304r and there is provided a slightclearance around member 300r to permit bleed back of fluid pressure inline 264r after actuation of the brakes has been terminated by thevehicle operator and after the pressure in line 261 r has just droppedbelow the minimum pressure set, i.e. 50

p The pressure in line 26lr and hence that delivered to output line 264rto the fluid separator 266r is dependent upon the actuation of controlvalve 252r. In general valve 252r is normally open to provide littlepressure at line 261r. However, valve 252r as actuated provides avariable restriction such that flow therethrough can be controllablythrottled. Since the fluid flow in the circuit will be generally at aconstant rate, the result will be an increase in pressure at line 261r.

The control valve 252r includes a main valve and orifice assembly 3l0rand a pilot valve and orifice assembly 3l2r. The main valve assembly3l0r includes a variable orifice 3l4r defined by a fixed tube 3l6r in apassage 3l8r, which is, fluid connected with passage 260r, and amovable, co-operating restriction surface 320r on a main valve member321r. Surface 320r is at the outer end of a nose portion 322r which isslidably supported in a bore 324r. Nose portion 322r is connected withan enlarged flange portion 326r which is secured at its radially outerextremity to a flexible diaphragm 328r. The main valve member 32lr issupported for axial motion via diaphragm 328r within a chamber and withthe diaphragm 328r dividing that chamber in chamber portions 330r and332r. As the main valve member 321r moves axially such that itsrestriction surface 320r moves towards or away from the tube 3l6r therestriction to fluid flow can be increased or decreased resulting in acorresponding increase or decrease in fluid pressure in passage 3l8r,and line 260r eventually resulting in corresponding variation in brakepressure via passage 261r, 264r, etc. The pressure acting on the mainvalve member 321r is controlled by the pilot valve and orifice assembly3l2r.

The pilot valve assembly 3l2r includes a passage 334r which extendsthrough a cap 336r which cap is utilized to close the downstream side ofthe chamber which houses the main valve member 326r. The pilot assembly312r also includes a restriction surface 338r on a cup shaped spoolmember 340r which is supported, in a manner to be described, for axialmovement relative to the passage 334r whereby the restriction surface338r will vary the restriction to fluid flow such that correspondingvariations in pressure in passage 324r will occur.

The spool member 340r is a part of an electromagnetic force motor 342rand has an actuating coil 344r wound thereon which coil is energized vialeads or conductors 40; thus the coil 344r will be energized via thesignal from the voltage controlled current source 204 in a manner to bedescribed.

The spool member 340r is supported for axial movement on a core member346r via a spindle 348r which extends through a central bore in core346r and is connected at one end to spool member 340r and terminates atits opposite end in an enlarged head portion 350r.

The spindle 348r is coupled to one end of the actuating plate 296 via aspring 352r which engages the head portion 350r at one end and a biasadjustment assembly 345r at its opposite end.

The bias assembly 354r is fixed to the actuating plat 296 and includes athreaded stud member 356r and locking nut 358r; the bias of the spring352r on spindle 348r can be set by adjusting the extension of studmember 356r relative to the actuating plate 296. With no fluid flow, thespring 352r will urge the restriction surface 338r to close passage334r. However, the low pressures present prior to brake actuation with anormal flow of fluid will be sufficient to move the spool 340rrearwardly to unseat the surface 338r.

The pilot valve assembly 312r is located in an enclosed chamber 360which has its fluid inlet via the passage 334r and its outlet to tankvia passage 258r. The main valve assembly 310r has its chamber portion330r connected to tank via passage 362r and chamber 360. The main valvemember 321r has a pair of fluid passages 364r and 366r which communicatethe inlet bore 324r with outlet passage 344r. The outlet passage 334r isalso fluid communicated with the chamber portion 332 of the main valveassembly 310r via ports 368r which are connected to path 364r. Thus themain valve member 32lr has tank pressure on one side of flange portion326r and the pressure in the passage 334r which is the pressuredeveloped by the pilot valve assembly 312r via the action of restrictionsurface 338r on the other side. In this way small changes in pressure atpassage 334r as caused by pilot valve assembly 3l2r will behydraulically amplified, via movement of main valve member 32lr toprovide a larger change in pressure in tube 3l6r and hence in thepressure delivered to the fluid separator 266r and then applied to therear brakes.

NORMAL BRAKE OPERATION For normal brake operation by the vehicleoperator the rod assembly 250 moves the actuating plate 296 with thismotion being resisted by the pedal feel spring 298. This motion will betransmitted to the spool member 340r via the spindle 348r resulting inthe restriction surface 338r being moved to vary the flow restrictionfrom passage 334r; since the system uses a constant flow source, theresult will be a change in pressure corresponding to the change in therestriction to flow. This change in pressure will be amplified by themain valve and orifice assembly 3l0r in the manner previously describedand will result in a pressure to the brakes generally proportional tothe input force as applied by the vehicle operator and which is reactedby the feel" spring 298.

FEED BACK MODIFICATION FIG. 6 depicts a modified form of the actuator 25of FIG. 5 in which a feed back of the actual brake pressure is utilizedto provide a brake pressure generally proportional to the input force.In FIG. 6 components similar to like components of FIG. 5 have beengiven the same numerical designation with the addition of postscript a.

Thus the actuator 25a has its rear brake control valve 252m whichincludes a bourdon tube 400r which has its movable end 402r operablyconnected to the actuating spindle 348m. The bourdon tube 400r receivesthe fluid pressure to the rear brakes via line 404r and thus pressure isreacted via the resultant force developed at the movable end 402r. Inthis case the opening of the pilot orifice will now reflect the actualapplied pressure to the brakes and the force applied by the vehicleoperator to provide the proper opening will also be a function of theactual brake pressure; thus the input force by the vehicle operator willvary as a function of the applied brake pressure.

SKID CONTROL OPERATION Skid control is provided by the electromagneticforce motor assembly 342r; this assembly includes a pair of annular,permanent magnets 370r polarized as indicated which are held between apair of pole pieces 372r and 374r with the magnet assembly beingpositioned via non-magnetic spacer rings 376r and 378. In a sense thespool 340r and coil 344r act like a voice coil and will move axially inresponse to the current applied to the coil 344r. In operation duringcontrolled operation the current to the coil 344r via conductor 42 willhave a magnitude varying in accordance with the magnitude of the errorsignal. This error signal current will cause the spool 340r to move toopen the passageway 344r with a force generally proportional to themagnitude of the current. The result will be a corresponding change inthe restriction, via restriction surface 338r, and hence a correspondingchange in the pressure at passage 334r. This will result incorresponding modulation of pressure to the brakes.

PUSH THROUGH OPERATION As noted, normally the brake actuation occursthrough the action of the control valve 252r; in the event, however, offailure of fluid supply of pump 254 the brakes can be manually actuatedby a push through feature. Considering FIGS. 4 and 5, the fluid actuator25 has the push through rod 300 which, when actuated, will causeactuation of the master cylinder 276 in a conventional manner. The rod300 is actuated through the pedal feel spring 298. The rod 300 has ahead portion 380 slidably supported in a sealed chamber 382 with its rodportion 384 extending through the chamber 382 and into the mastercylinder 276. The chamber 382 is normally filled with fluid from thesystem of pump 254 and the fluid is trapped there and hence preventsmovement of push through rod 300. The chamber 382 is connected to afluid path 386 which is connected to tank 256 via a latch pistonassembly 388. A passage 390 communicates the chamber 382 at one side ofa differential piston 392 which has a seal 394 engageable with thepassage 390 to close it. The opposite side of the piston 392 isconnected to the outlet line 260 from pump 254 via passage 396. The areaof seal 394 exposed to pressure in chamber 382 is small compared to thearea of the piston 392 exposed to pump pressure and hence the piston 392will normally be actuated to hold passage 390 closed keeping the fluidtrapped in chamber 382 thereby preventing movement of push through rod300. In the event of loss of pressure of pump 254, the pressure inchamber 382 has caused by the vehicle operator through rod assembly 250and spring 298 and the bias of the spring under the valve piston 392will cause the valve piston 392 to unseat seal 394 permitting thetrapped fluid to flow to tank 256 via passage 386 and resulting in pushthrough rod 300 moving to actuate the brakes via fluid passage 380r,

valve 282r and line 274r.

The head portion 380 of rod 300 has a port 398 which cooperates with acup seal 400 such that upon re-establishment of fluid pressure in thecircuit of pump 254, return motion of the push-through rod 300 willresult in chamber 384 being filled with fluid in chamber 360 via port398 and past the lip of the cup seal 400.

MODIFIED PUSH THROUGH FIG. 7 depicts a modified form of the actuator 25of FIG. 7 in which a mechanical latch is used to hold off push throughexcept in cases of failure of the supply of the pump. In FIG. 7,components similar to like components of F IG. 5 have been given thesame numerical designation with addition of postscript b.

Thus the actuator 25b has the push through rod 30% provided with anannular groove 310; latch piston 388b has actuating piston 392b whichhas a rod 412 which is normally located in groove 410 to hold rod 300kfrom moving. Piston 392b is held in the latched position by fluidpressure from the pump via line 396b. In the event of loss of fluidpressure at the pump, the rod 412 (and hence piston 392b) will be movedout of groove 410 and brake pressure can then be applied directly by thevehicle operator.

MODIFIED FLUID SEPARATOR FIG. 4 A shows a modified form 266r' of fluidseparator for the system of FIG. 4. Separator 266r can be similar to asingle cylinder master cylinder and has a piston 265r reciprocablymounted in a housing 267r with one side of piston 265r connected to thecontrolled fluid pressure via line 264r and with the other side ventedto atmosphere via outlet 269r. The piston 265r has a plunger 27lractuable on a piston 273r in a master cylinder like assembly 275r. Thepiston 273r is normally held deactuated via spring 277r from acting onfluid in chamber 279r which has an outlet connected to output line 274rvia the valve 282r. A reservoir 281r is connected to chamber 279r aswith a conventional master cylinder.

FRONT BRAKE SYSTEM In the discussion of the fluid actuator 25 generallyonly the rear brake system has been shown and described in FIG. 4 and,while the front brake system has been shown in FIG. 5, only the rearbrake system has been described. With regard to the above the frontbrake system is substantially identical to the rear brake system andgenerally the description of the rear brake system also would apply tothe front system. Where a numeral designation has the letter r addedthis indicates that it applies to the rear brake system and it alsodenotes that a similar component operating in the same manner isincorporated in the front brake system.

Looking again to FIG. 5, note that the front control valve 252f isconnected to the actuating plate 296 and hence the front and rear brakeswill be actuated in a similar manner together. However, the springcoupling via spring 352f permits the front control valve 252f to becontrollably operated independently of the rear brake control valve 252rand vice versa.

Note that with the system as shown and described an error signal isgenerated having a magnitude which is generally proportional to themagnitude of the error in wheel acceleration or deceleration relative todesired references and correction and/or modulation is provided in thebrake pressure with the magnitude of this correction being generallyproportional to the magnitude of the integral of error signal (Aw). Alsonote that initial control (in mode B) is made to control wheeldeceleration with this control being initiated at a wheel decelerationreference which for high p. surfaces may be less than that magnitudewhich is indicative of an incipient skid condition. Thus the firstcorrection is made regardless of whether a skid condition exists or notand it is possible to have mode B operation with no subsequent mode Aoperation. However, one of the significant advantages of mode B is aftera mode A operation the pressure will not be applied excessively (i.e.,full on) but rather will be applied to provide the desired controlleddeceleration rate, i.e., 3 5 gs. For an automotive application, this wasattained with the brake pressure modulated to provide pressure relief ofaround 30 percent of the applied pressure for an incipient skidcondition on a high p. surface. This contrasts to on off type systems inwhich the brake pressure is almost completely relieved. It should alsobe noted that the controller 25 generally will provide brake pressure tothe brakes which is proportional to the input force as applied by thevehicle operator.

While it will be apparent that the preferred embodiments of theinvention disclosed are well calculated to fulfill the objects abovestated, it will be appreciated that the invention is susceptible tomodification, variation and change without departing from the properscope or fair meaning of the invention.

What is claimed is:

1. A control system for the brake system for hydraulic actuated brakesfor at least one wheel of a wheeled vehicle comprising: actuator meansactuable by the vehicle operator for controlling the hydraulic pressureto the brakes, said actuator means comprising valve means includingpilot valve means actuatable by the vehicle operator for continuouslyvarying an input hydraulic pressure over a first range of hydraulicpressures in accordance with continuous variations in actuation by saidvehicle operator, and main valve means responsive to said inputhydraulic pressure for continuously varying an output hydraulic pressureover a second range of hydraulic pressures in accordance with continuousvariations in said input hydraulic pressure, said second range ofhydraulic pressures being substantially greater than said first range ofhydraulic pressures so that said hydraulic output pressure represents apressure amplification of the hydraulic input pressure.

2. The system of claim 1 including means for sensing the pressure to thebrakes of said at least one wheel and feeding back a force to said valvemeans having a magnitude indicative of the magnitude of said forcewhereby the actuation by the vehicle operator will be responsive to saidforce.

3. The system of claim 1 including skid control means for providing acontrol signal in response to an incipient skid condition, said pilotvalve means including control means responsive to said control signalfor varying said input hydraulic pressure variations.

4. The system of claim 3 with said pilot valve means including a firstvariable orifice having first fixed and first movable members and withsaid control means including said first movable member.

5. The system of claim 3 with said pilot valve means including anelectrically actuated force motor actuable in response to said controlsignal.

6. The system of claim 5 with said pilot valve means including a firstvariable orifice having first fixed and first movable members, saidforce motor actuatable to move said first movable member.

7. The system of claim 5 including linkage means operably connected tosaid pilot valve means for controlling actuation of said pilot valvemeans in response to actuation by the vehicle operator, said force motorbeing operable for actuation of said pilot valve means independently ofactuation of said linkage means.

8. The system of claim 7 with said pilot valve means including a firstvariable orifice having first fixed and first movable members, and forcemotor having a movable plunger which is a part of said first movablemember.

9. A control system for the brake system for fluid actuated front andrear wheel brakes of an automotive vehicle, comprising: actuator meansactuable by the vehicle operator for controlling the pressure to thefront and rear wheel brakes, said actuator means comprising valve meansactuatable by the operator to provide pressure amplification of thefluid pressure from the source, said valve means including first mainvalve means actuatable in response to first input pressure variation forproviding amplified outlet pressure variations for the brakes of atleast one wheel of one set of the front and rear wheel brakes and firstpilot valve means actuatable by the vehicle operator for providing saidfirst input pressure variations, said valve means further includingsecond main valve means actuable in response to second input pressurevariations for providing amplified outlet pressure variations for thebrakes of at least one wheel of the other set of the front and rearwheel brakes and second pilot valve means actuatable by the vehicleoperator for providing said second input pressure variations.

10. The system of claim 9 with said first and second pilot valve meansbeing actuable independently of each other.

11. The system of claim 10 including means for sensing the pressure tothe brakes of said at least one wheel of said one set of brakes andfeeding back a force to said first pilot valve means havinga magnitudeindicative of the magnitude of said force.

12. The system of claim 10 including skid control means for providingcontrol signals in response to an incipient skid condition, said firstpilot valve means including first control means responsive to a first ofsaid control signals for varying said first input pressure variation.

13. The system of claim 12 with said second pilot valve means includingsecond control means responsive to a second of said control signals forvarying said second input pressure variation.

14. The system of claim 12 with said skid control means providing saidfirst control signal having a magnitude varying in accordance with adeterminable degree of variation of brake pressure to provide skidcontrol, said first control means responsive to said first controlsignal for varying said first input pressure variation an amount inaccordance with the magnitude of said first control signal.

15. The system of claim 14 with said amount being generally proportionalto the magnitude of said control signal,

16. The system of claim 10 including flow divider means operable from asource of fluid pressure for providing a first supply of fluid under asubstantially constant flow rate to said first pilot and main valvemeans and a second supply of fluid under substantially constant flowrate to said second pilot and main valve means.

17. The system of claim 16 including a first holdoff valve connected tothe outlet of said first main valve means for blocking pressurevariation at that outlet to the associated brakes until a preselectedminimum pressure is attained.

18. The system of claim 17 including a second holdoff valve connected tothe outlet of said second main valve means for blocking pressurevariation at that outlet to the associated brakes until a preselectedminimum pressure is attained.

19. The system of claim 16 including first fluid separator meansactuable in response to fluid pressure of said first supply of fluiddeveloped by said first main valve means to apply pressure to the brakesby a first separate supply of fluid.

20. The system of claim 19 including a master cylinder operable withsaid first separate supply of fluid, said actuator means including pushthrough means for sensing the pressure from the source of said first andsecond supply of fluid for normally preventing actuation of said mastercylinder and permitting its actuation in response to a loss of pressurefrom the source.

21. In a brake system for a wheeled vehicle having brakes actuable by afirst fluid and in which the system includes a source of fluid pressurefor a second fluid, a controller actuable by said second fluid forcontrolling the pressure of the first fluid to the brakes of at leastone wheel of the vehicle, comprising: a pilot valve having a firstvariable orifice, a main valve having a second variable orifice, firstfluid passage means connecting said pilot valve and said main valve withsaid main valve being actuated in response to pressure variations atsaid first variable orifice to provide a corresponding variation of saidsecond variable orifice resulting in an amplification of the pressurevariations at the inlet to said main valve, a master cylinder operablewith the first fluid, second fluid passage means connecting the outputof said master cylinder to the brakes, said second fluid passage meansincluding a blocking valve normally blocking said second fluid passagemeans and being responsive to loss of pressure at the source for openingsaid second fluid passage means, linkage means operably connected tosaid pilot valve for controlling said first variable orifice in responseto actuation by the vehicle operator, said linkage means including apush-through rod connected to receive the force of the vehicle operatorduring brake application and connected to said master cylinder, hold-offmeans for normally preventing said push-through rod from acting on saidmaster cylinder and including a valve member responsive to a loss ofpressure at the source for permitting said push-through rod to act onsaid master cylinder.

22. A control system for the brake system for fluid actuated brakes forat least one wheel of a wheeled vehicle comprising: actuator meansactuable by the vehicle operator for controlling the fluid pressure tothe brakes, said actuator means comprising valve means including pilotvalve means actuatable by the vehicle operator for continuously varyingan input fluid pressure over a first range of fluid pressures inaccordance with continuous variations in actuation by said vehicleoperator, and main valve means responsive to said input fluid pressurefor continuously varying an output fluid pressure over a second range offluid pressures in accordance with continuous variations in said inputfluid pressure, said second range of fluid pres sures beingsubstantially greater than said first range of fluid pressures so thatsaid output fluid pressure represents a pressure amplification of thefluid input pressure, said control system further comprising skidcontrol means for providing a control signal in response to an incipientskid condition, said pilot valve means including control meansresponsive to said control signal for varying said input fluid pressurevariations.

23. The system of claim 22 with said pilot valve means including a firstvariable orifice having first fixed and first movable members and withsaid control means including said first movable member.

24. The system of claim 22 with said pilot valve means including anelectrically actuated force motor actuatable in response to said controlsignal.

25. The system of claim 24 with said pilot valve means including a firstvariable orifice having first fixed and first movable members, saidforce motor actuable to move said first movable member.

26. The system of claim 24 including linkage means operably connected tosaid pilot valve means for controlling actuation of said pilot valvemeans in response to actuation by the vehicle operator, said force motorbeing operable for actuation of said pilot valve means independently ofactuation of said linkage means.

27. The system of claim 26 with said pilot valve means including a firstvariable orifice having first fixed and first movable members, and forcemotor having a movable plunger which is a part of said first movablemember.

1. A control system for the brake system for hydraulic actuated brakesfor at least one wheel of a wheeled vehicle comprising: actuator meansactuable by the vehicle operator for controlling the hydraulic pressureto the brakes, said actuator means comprising valve means includingpilot valve means actuatable by the vehicle operator for continuouslyvarying an input hydraulic pressure over a first range of hydraulicpressures in accordance with continuous variations in actuation by saidvehicle operator, and main valve means responsive to said inputhydraulic pressure for continuously varying an output hydraulic pressureover a second range of hydraulic pressures in accordance with continuousvariations in said input hydraulic pressure, said second range ofhydraulic pressures being substantially greater than said first range ofhydraulic pressures so that said hydraulic output pressure represents apressure amplification of the hydraulic input pressure.
 2. The system ofclaim 1 including means for sensing the pressure to the brakes of saidat least one wheel and feeding back a force to said valve means having amagnitude indicative of the magnitude of said force whereby theactuation by the vehicle operator will be responsive to said force. 3.The system of claim 1 including skid control means for providing acontrol signal in response to an incipient skid condition, said pilotvalve means including control means responsive to said control signalfor varying said input hydraulic pressure variations.
 4. The system ofclaim 3 with said pilot valve means including a first variable orificehaving first fixed and first movable members and with said control meansincluding said first movable member.
 5. The system of claim 3 with saidpilot valve means including an electrically actuated force motoractuable in response to said control signal.
 6. The system of claim 5with said pilot valve means including a first variable orifice havingfirst fixed and first movable members, said force motor actuatable tomove said first movable member.
 7. The system of claim 5 includinglinkage means operably connected to said pilot valve means forcontrolling actuation of said pilot valve means in response to actuationby the vehicle operator, said force motor being operable for actuationof said pilot valve means independently of actuation of said linkagemeans.
 8. The system of claim 7 with said pilot valve means including afirst variable orifice having first fixed and first movable members, andforce motor having a movable plunger which is a part of said firstmovable member.
 9. A control system for the brake system for fluidactuated front and rear wheel brakes of an automotive vehicle,comprising: actuator means actuable by the vehicle operator forcontrolling the pressure to the front and rear wheel brakes, saidactuator means comprising valve means actuatable by the operator toprovide pressure amplification of the fluid pressure from the source,said valve means including first main valve means actuatable in responseto first input pressure variation for providing amplified outletpressure variations for the brakes of at least one wheel of one set ofthe front and rear wheel brakes and first pilot valve means actuatableby the vehicle operator for providing said first input pressurevariations, said valve means further including second main valve meansactuable in response to second input pressure variations for providingamplified outlet pressure variations for the brakes of at least onewheel of the other set of the front and rear wheel brakes and secondpilot valve means actuatable by the vehicle operator for providing saidsecond input pressure variations.
 10. The system of claim 9 with saidfirst and second pilot valve means being actuable independently of eachother.
 11. The system of claim 10 including means for sensing thepressure to the brakes of said at least one wheel of said one set ofbrakes and feeding back a force to said first pilot valve means having amagnitude indicative of the magnitude of said force.
 12. The system ofclaim 10 including skid control means for providing control signals inresponse to an incipient skid condition, said first pilot valve meansincluding first control means responsive to a first of said controlsignals for varying said first input pressure variation.
 13. The systemof claim 12 with said second pilot valve means including second controlmeans responsive to a second of said control signals for varying saidsecond input pressure variation.
 14. The system of claim 12 with saidskid control means providing said first control signal having amagnitude varying in accordance with a determinable degree of variationof brake pressure to provide skid control, said first control meansresponsive to said first control signal for varying said first inputpressure variation an amount in accordance with the magnitude of saidfirst control signal.
 15. The system of claim 14 with said amount beinggenerally proportional to the magnitude of said control signal.
 16. Thesystem of claim 10 including flow divider means operable from a sourceof fluid pressure for providing a first supply of fluid under asubstantially constant flow rate to said first pilot and main valvemeans and a second supply of fluid under substantially constant flowrate to said second pilot and main valve means.
 17. The system of claim16 including a first holdoff valve connected to the outlet of said firstmain valve means for blocking pressure variation at that outlet to theassociated brakes until a preselected minimum pressure is attained. 18.The system of claim 17 including a second hold-off valve connected tothe outlet of said second main valve means for blocking pressurevariation at that outlet to the associated brakes until a preselectedminimum pressure is attained.
 19. The system of claim 16 including firstfluid separator means actuable in response to fluid pressure of saidfirst supply of fluid developed by said first main valve means to applypressure to the brakes by a first separate supply of fluid.
 20. Thesystem of claim 19 including a master cylinder operable with said firstseparate supply of fluid, said actuator means including push throughmeans for sensing the pressure from the source of said first and secondsupply of fluid for normally preventing actuation of said mastercylinder and permitting its actuation in response to a loss of pressurefrom the source.
 21. In a brake system for a wheeled vehicle havingbrakes actuable by a first fluid and in which the system includes asource of fluid pressure for a second fluid, a controller actuable bysaid second fluid for controlling the pressure of the first fluid to thebrakes of at least one wheel of the vehicle, comprising: a pilot valvehaving a first variable orifice, a main valve having a second variableorifice, first fluid passage means connecting said pilot valve and saidmain valve with said main valve being actuated in response to pressurevariations at said first variable orifice to provide a correspondingvariation of said second variable orifice resulting in an amplificationof the pressure variations at the inlet to said main valve, a mastercylinder operable with the first fluid, second fluid passage meansconnecting the output of said master cylinder to the brakes, said secondfluid passage means including a blocking valve normally blocking saidsecond fluid passage means and being responsive to loss of pressure atthe source for opening said second fluid passage means, linkage meansoperably connected to said pilot valve for controlling said firstvariable orifice in response to actuation by the vehicle operator, saidlinkage means including a push-through rod connected to receive theforce of the vehicle operator during brake application and connected tosaid master cylinder, hold-off means for normally preventing saidpush-through rod from acting on said master cylinder and including avalve member responsive to a loss of Pressure at the source forpermitting said push-through rod to act on said master cylinder.
 22. Acontrol system for the brake system for fluid actuated brakes for atleast one wheel of a wheeled vehicle comprising: actuator means actuableby the vehicle operator for controlling the fluid pressure to thebrakes, said actuator means comprising valve means including pilot valvemeans actuatable by the vehicle operator for continuously varying aninput fluid pressure over a first range of fluid pressures in accordancewith continuous variations in actuation by said vehicle operator, andmain valve means responsive to said input fluid pressure forcontinuously varying an output fluid pressure over a second range offluid pressures in accordance with continuous variations in said inputfluid pressure, said second range of fluid pressures being substantiallygreater than said first range of fluid pressures so that said outputfluid pressure represents a pressure amplification of the fluid inputpressure, said control system further comprising skid control means forproviding a control signal in response to an incipient skid condition,said pilot valve means including control means responsive to saidcontrol signal for varying said input fluid pressure variations.
 23. Thesystem of claim 22 with said pilot valve means including a firstvariable orifice having first fixed and first movable members and withsaid control means including said first movable member.
 24. The systemof claim 22 with said pilot valve means including an electricallyactuated force motor actuatable in response to said control signal. 25.The system of claim 24 with said pilot valve means including a firstvariable orifice having first fixed and first movable members, saidforce motor actuable to move said first movable member.
 26. The systemof claim 24 including linkage means operably connected to said pilotvalve means for controlling actuation of said pilot valve means inresponse to actuation by the vehicle operator, said force motor beingoperable for actuation of said pilot valve means independently ofactuation of said linkage means.
 27. The system of claim 26 with saidpilot valve means including a first variable orifice having first fixedand first movable members, and force motor having a movable plungerwhich is a part of said first movable member.